It has been well known for years that chlorine is biocidally active, being able to kill bacteria and algae. The use of chlorine to kill bacteria in municipal water supplies is well known. Chlorine has also been used in swimming pools to kill bacteria and algae. Chlorine is most effective under mildly alkaline conditions, e.g. at a pH of about 7 to about 8, as is well known.
Swimming pools have normally been treated by adding chlorine in a chemical form, either gaseous chlorine or sodium or calcium hypochlorite, to the water in the pool. The chlorine is stored in bottles under pressure, and a rather elaborate and expensive system for adding chlorine from bottles to a pool is required. Furthermore, chlorine is toxic if released into the atmosphere. The use of bottled chlorine can be justified only in large pools where a skilled operator controls the addition of chlorine. Calcium and sodium hypochlorite are easier to use, and are better suited than chlorine for use in small pools or in pools where no skilled operator is available. However, sodium and calcium hypochlorite tend to degrade in the presence of moisture if stored for any substantial length of time.
Generally, the chlorine or hypochlorite is added manually, in response to a perceived need for addition by the pool owner or operator. The addition of chlorine tends to become rather haphazard, particularly in home swimming pools or other swimming pools where there is no operator. Frequently one will fail to add chlorine as needed with the result that algae may appear and bacterial contamination may reach an undesirably high level.
Automatic halogen and pH control systems for swimming pools are known, as shown for example in U.S. Pat. No. 4,550,011 to McCollum, but automatic systems have not enjoyed widespread use.
Swimming pool chlorination systems employing an electrolytic cell for generation of chlorine are well known. The source of chlorine may be either sodium chloride solution (brine) or hydrochloric acid solution. Examples of systems employing brine include U.S. Pat. No. 3,669,857 to Kirkham et al and U.S. Pat. No. 4,129,493 to Tighe et al. Systems employing hydrochloric acid include that shown in U.S. Pat. No. 3,351,542 to Oldershaw et al. Cells using sodium chloride are preferred because of the lower cost of sodium chloride as compared to hydrochloric acid. Also, cells using hydrochloric acid are not as safe as those using sodium chloride.
The principal chemical reactions which typically take place in an electrolytic cell for generation of chlorine from brine are shown in equations 1 and 2 below. EQU 2NaCl+2H.sub.2 O=2NaOH+H.sub.2 +Cl.sub.2 ( 1) EQU 2H.sub.2 O=2H.sub.2 +O.sub.2 ( 2)
The primary reaction taking place is the electrolysis of brine, as shown by equation (1). Sodium hydroxide is co-produced with chlorine, as is well known. Some electrolysis of water takes place simultaneously; this secondary or side reaction is shown in equation (2). This results in the production of oxygen and hydrogen.
The ionic reactions taking place at the anode of the chlorine cell are shown in equations (3) and (4) below. Equation (5) shows the reaction taking place in the cathode. EQU 4 Cl.sup.- =2Cl.sub.2 +4e.sup.- ( 3) EQU 4 OH.sup.- =2H.sub.2 O+O.sub.2 +4e.sup.- ( 4) EQU 4 H.sub.2 O+4e.sup.-=4 OH.sup.- 2H.sub.2 ( 5)
Chlorine and caustic soda, when introduced into a body of water such as a swimming pool, interact under mildly alkaline conditions according to equations (6) and (7) below to form a mixture of hypochlorous acid and sodium hypochlorite in solution. EQU Cl.sub.2 +2NaOH=2NaOCl+H.sub.2 O (6) EQU NaClO+H.sub.2 O=HClO+Na.sup.+ +OH.sup.- ( 7)
Reduction of this specie takes place according to equation (8): EQU HClO+OH.sup.- +2e.sup.- =Cl.sup.- +2 OH.sup.- ( 8)
When this hypochlorous acid reacts with the organic matter, the oxidation process can be written as: EQU OR(r)=OR(ox)+n e (9)
where the OR(r) and OR(ox) indicates the reduced and oxidized organic matter, respectively.
Swimming pools may be broadly classified as either salt water pools and fresh water pools. Electrolytic cells can be used to generate chlorine for either type of pool. Cells for salt water pools typically have a single body of electrolyte, with no separator between the anode and the cathode. Cells for fresh water pools generally have an ion-selective membrane which divides the cell interior into anolyte and catholyte compartments.
Membrane type cells for swimming pool chlorination systems are shown for example in the Kirkham et al and Tighe et al patents cited above. The membrane separates the cell into anolyte and catholyte compartments. The membrane is typically impermeable to electrolyte and to anions, such as chloride and hydroxyl, while permitting migration of cations such as sodium. The principal advantage of a membrane is that sodium hydroxide produced in the catholyte compartment is substantially pure, since chloride ion, which otherwise would be a contaminant, cannot migrate from the anolyte compartment to the catholyte compartment. The disadvantages of membranes is that they are very expensive and they may fail suddenly and catastrophically due to cracking. A porous separator may be placed next to the membrane, as shown for example in U.S. Pat. No. 3,669,857 cited supra, in order to support the membrane and thereby protect the membrane (which is structurally weak) from fatigue and cracking.
Small chlorine cells employing a porous separator, e.g. a diaphragm, are known, as shown for example in Bianchi, U.S. Pat. No. 4,496,452. Kirkham et al, U.S. Pat. No. 3,669,857 cited supra, mentions a porous diaphragm as a less desirable alternative membrane. Although porous separators or diaphragms are widely used in electrolytic cells for commercial production of chlorine and caustic soda, cells employing porous separators are not used in swimming pool chlorination systems as far as applicant is aware.
Chlorine cells for swimming pools may have other disadvantages as well. For example, unreacted sodium chloride (spent brine) is usually added to the swimming pool along with chlorine gas which is generated, making the pool water undesirably salty. Note U.S. Pat. Nos. 3,669,857 and 4,129,493 (both cited previously) in this regard. Frequently, hydrogen gas is vented to the atmosphere as shown for example in U.S. Pat. No. 4,136,005 to Persson et al. This creates a safety hazard, particularly if anyone smokes in the area of the cell. Furthermore, there is typically no automatic control other than a timer to control the generation of chlorine. While a timer as the sole control may work reasonably well as long as chlorine demand remains reasonably constant and the timing cycle is properly set for that demand, systems relying on a timer as the sole control cannot compensate for excessive or diminished chlorine demand, the former occurring for example during extremely hot weather or when the pool is heavily used. Some swimming pool chlorination systems employing an electrolytic cell also have means for sensing the oxidation-reduction potential (ORP) of the pool water, but none as far as applicant is aware includes means for sensing both the ORP (which is a measure of chlorine content) and the pH of pool water with associated means for controlling the addition of chlorine and acid accordingly.